An electric motor is typically used to drive the shaft of a compressor used in heating ventilation and air conditioning (HVAC) systems. The electric motor can be an alternating current (AC) capacitor-start capacitor-run (CSCR) motor. CSCR motors can conveniently run on single phase AC power, such as between the lines of a standard 230 V AC commercial or residential power system. A CSCR AC motor has two windings, a “run” winding and a “start” winding. Motion is created in the rotor of the motor by causing a phase difference between the AC electrical currents in the two windings. This phase difference is caused by introducing a capacitor in series with one of the windings.
A CSCR motor is started by momentarily introducing a larger capacitance to provide a high mechanical starting torque. Once the rotor speed is up to a desired speed, the starting capacitance is typically switched out of the circuit by a relay, leaving a smaller value run capacitor in circuit to establish the correct phase relationship between the start and the run winding for continuous operation. Typically, the voltage across one of the windings, usually the start winding, is monitored by use of a potential relay to determine when to open the start capacitor relay. If the start capacitor relay is opened too soon before the motor has achieved a sustainable shaft rotation speed, the motor will stall when the start capacitor relay is opened. If the start capacitor relay is opened too late, the winding currents and voltages can become excessively high potentially causing high temperatures, damaging mechanical stresses, and insulation breakdown. The accuracy of potential relays is only about 20 VAC which limits the ability to accurately time the opening of the start capacitor contactor. What is needed is a system to more accurately match the opening of the motor start relay to a desired mechanical rotor condition. The mechanical rotor condition can also be influenced by line voltage, ambient temperature, motor temperature, and compressor condition, such as how recently the compressor was run. Therefore, there is also a need for a system to vary the operation of the motor start capacitor relay to compensate for factors such as line voltage, ambient temperature, motor temperature, or compressor operating conditions.
Another desirable feature of CSCR motors is that they can be operated as reversible motors where the shaft can be powered to turn clockwise or counter clockwise. The rotation direction can be forced by setting the phase of the current in the run winding to lead or lag the phase of the current in the start winding. This can be accomplished by placing a capacitor in series with one or the other winding and directly connecting the remaining winding across the powering voltage (typically two lines of a three phase power source). Such switching can be accomplished by using two sets of contacts, usually in the form of contactors, one for a forward rotation and one for reverse motor shaft rotation.
Two directions of rotational direction are particularly desirable for driving the compressor shaft of modern compressors. Such compressors can make use of a compressor shaft technology where when the shaft rotated in different directions, the compressor generates two different rates of compression. The principle of operation is that a mechanical mechanism operates fewer pistons when rotated on one direction than when the direction of rotation is reversed. Typically a forward rotation operates two pistons, while a reverse rotation causes one piston to operate. The problem is that the optimal conditions for opening the starting relay are different for either direction because of the different mechanical loading. Therefore what is also needed is a system to vary the operation of the capacitor start relay as a function of desired rotational direction.
Yet another problem in CSCR motor operation is the reliability of the motor start capacitor relay. Even if the motor start capacitor relay is operated at the correct time for the correct mechanical rotor condition, an arc caused by opening the motor start capacitor relay can reduce the life of the relay or even cause a contact failure that can destroy the capacitor or motor by then failing to remove the start capacitor from the circuit following an otherwise successful motor start. Therefore what is also needed is a system to open the motor start capacitor relay so as to cause minimal electrical stress to the electrical contacts of the relay.